The present invention relates to a method of producing a thin-film photovoltaic device, and more particularly to a method of producing a thin-film photovoltaic device having a rear electrode excellent in strength of bonding with a photovoltaic conversion unit.
Recently, thin-film photovoltaic devices having a semiconductor thin film as a photovoltaic conversion film, such as solar cells, have been developed briskly. Generally, such thin-film photovoltaic devices have, on an insulating substrate, a rear electrode, a thin-film photovoltaic conversion unit having a pin or nip junction structure and provided in front of the rear electrode, and a transparent front electrode provided in front of the photovoltaic conversion unit. These thin-film photovoltaic devices have been expected to find applications in various fields such as optical sensors, in addition to solar cells.
Various improvements have been proposed to enhance the photovoltaic conversion amount of the thin-film photovoltaic devices. For example, it has been proposed to form the rear electrode from silver. Silver has a very high light-reflectivity. The light entering the photovoltaic conversion unit repeats reflection between the high light-reflectivity silver rear electrode and the transparent front electrode. The repeated reflection increases light absorption by the photovoltaic conversion layer, thus enhancing the photovoltaic conversion amount.
Further, a number of the photovoltaic devices have recently been proposed in which a transparent electrically conductive layer made of, e.g., zinc oxide (hereinafter referred to as transparent conductive rear layer) is disposed between a light-reflective metallic layer and a silicon-based photovoltaic conversion unit (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 3-99477, and Jpn. Pat. Appln. KOKAI Publication No. 7-263731). By disposing the transparent conductive rear layer between the light-reflective metallic layer of the rear electrode and the photovoltaic conversion unit, the thermal strain, which may be caused by a difference in thermal expansion coefficient between them, is alleviated, and the diffusion of the metal atoms into the silicon-based photovoltaic conversion unit is prevented. As a result, not only the yield and reliability of the photovoltaic device products are increased, but also the optical sensitivity of the devices is enhanced, leading to improved photovoltaic characteristics.
However, silver has an inferior bonding strength with respect to thin-film semiconductors and ceramics. The bonding strength of silver is not so great that silver can be practically used as the material of the rear electrode in the thin-film photovoltaic devices. As described above, if the transparent conductive rear layer is provided between the silver rear electrode and the photovoltaic conversion unit, the bonding strength may be slightly increased. However, such an increase in the bonding strength is not practically sufficient at all, and still the above-noted effects, which the transparent conductive rear layer inherently has, cannot be utilized sufficiently.
Accordingly, an object of the present invention is to provide a method of producing a thin-film photovoltaic device having a silver-based rear electrode excellent in bonding strength with respect to the transparent conductive rear layer.
The present inventors have conducted extensive studies in an attempt to achieve the above-noted object, during which they have paid attention to a technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-107225 filed by the assignee of the present application. According to this technique, firstly, a transparent electrode layer, a thin-film semiconductor layer and a first transparent conductive metallic compound layer are successively formed on an insulating transparent substrate. Then, the substrate is passed first over a first plasma region of transparent conductive metallic compound and then a second plasma region of metal, which are formed in a sputter chamber. As a result, a second transparent conductive metallic compound layer and a metal layer are formed on the first transparent conductive metallic compound layer. The first transparent conductive metallic compound layer and the metallic layer are bonded with each other firmly through the second transparent conductive metallic compound layer, thus forming an integral rear electrode.
The present inventors have studied this technique, and found out that (1) if silver is selected as the material of the metal layer, zinc oxide is optimum as the material of the first transparent conductive metallic compound layer in view of bondability with silver, (2) zinc oxide is also optimum as the material of the second transparent conductive metallic compound layer which acts as a bonding layer, and (3) more importantly, a pressure inside the sputter chamber during forming the zinc oxide layer and the silver layer is closely related to the bonding strength between the zinc oxide layer and the silver layer. The present invention is based on these findings.
According to the present invention, there is provided a method of producing a thin-film photovoltaic device which has a rear electrode including a transparent conductive rear layer comprising zinc oxide, and a light-reflective metallic layer comprising silver and provided in the rear of the conductive rear layer. The method according to the invention comprises the steps of forming, on a substrate, a transparent front electrode, a thin-film photovoltaic conversion unit provided in the rear of the front electrode, and the conductive rear layer provided in the rear of the photovoltaic conversion unit. In forming the light-reflective metallic layer, a first plasma region comprising fine particles of zinc oxide and a second plasma region comprising fine particles of silver are formed in a chamber at a sputtering gas pressure of about 0.1 to about 0.27 Pa. Then, the substrate is passed over the first and second plasma regions formed in the chamber to form a bonding layer comprising the zinc oxide and a light-reflective metallic layer comprising the silver, thereby providing the rear electrode comprising the transparent conductive rear layer, the bonding layer and the light-reflective metallic layer.
Where a thin-film photovoltaic device having a thin-film photovoltaic conversion unit of a pin junction structure is produced according to the present invention, the transparent front electrode, thin-film photovoltaic conversion unit and transparent conductive rear layer are formed successively on the substrate, and then the substrate is passed first over the first plasma region and then over the second plasma region to form the bonding layer and the light-reflective metallic layer successively over the transparent conductive rear layer.
Where a thin-film photovoltaic device having a thin-film photovoltaic conversion unit of a nip junction structure is produced according to the present invention, the substrate is passed first over the second plasma region and then over the first plasma region to form the light-reflective metallic layer and the bonding layer on the substrate, and thereafter the transparent conductive rear layer, thin-film photovoltaic conversion unit and transparent front electrode are formed successively over the light-reflective metallic layer.
Preferably, the first plasma region is formed by applying a discharge power to a zinc oxide sputter target which may contain a dopant, while the second plasma region is formed by applying a discharge power to a silver sputter target. In such a case, it is preferred that the power density of the discharge power applied to the zinc oxide sputter target is about 0.1 to 4 W/cm2, and the power density of the discharge power applied to the silver target is about 1 to about 20 W/cm2.
In the present invention, the first plasma region and the second plasma region may be formed such that they partially overlap each other to such an extent that the reflectivity of the light-reflective metallic layer formed at the second plasma region is not substantially decreased.
In a preferred embodiment, the zinc oxide-based transparent conductive rear layer is formed by sputtering a zinc oxide target at a temperature of 150xc2x0 C. or more and a sputtering gas pressure of 1 Pa or more.
In another preferred embodiment, the thin-film photovoltaic conversion unit has an n-type semiconductor layer composed of silicon and containing phosphorus, as an impurity, in an amount of 2 to 8 atomic percent based on the silicon, and the transparent conductive rear layer is composed of zinc oxide and contains aluminum in an amount of 1 to 5% by weight.
In this specification, the term xe2x80x9cfrontxe2x80x9d refers to a light incident side of the device, while the term xe2x80x9crearxe2x80x9d refers to the opposite side of the device. Further, the terms xe2x80x9cnip junction structurexe2x80x9d and xe2x80x9cpin junction structurexe2x80x9d refer to the order of a p-type semiconductor layer, an i-type semiconductor layer and an n-type semiconductor layer from the substrate. More specifically, the nip junction structure is a structure having an n-type semiconductor layer, an i-type semiconductor layer and a p-type semiconductor layer which are disposed in this order on the substrate. On the other hand, the pin junction structure is a structure having a p-type semiconductor layer, an i-type semiconductor layer and an n-type semiconductor layer in this order on the substrate.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.